Gas-producing compositions



3,073,730 GAS-PRUDUQWG QQMPOSHTEQNS Frederick H. Doe, 80 Mouhswood Ave., Waltham Abbey,

Eiigiaud; Philip B. Freeman, 76 Waltham Way, Citingford, London E. 4, England; Colin G. Lawson, 32 Horsley Road, Chingford, London E. 4, England; Barry H. Newman, 82. Windmill Lane, Cheshunt, England; Charles H. Peers, 15 Sussex Way, Cochfosters, Barrret, England; and George H. 5. Young, 123 Teuuiswood Road, Enlield, England No Drawing. Filed Get. 9, 1957, Scr. No. 689,245 Claims priority, application Great Britain (Pet. 10, 1956 '(Ziaims. (Cl. 149-49) The present invention relates to gas-producing compositions which may be used in rocket motors, engine starter cartridges, fluid ejection systems and like devices in which gas is required to be generated under pressure.

The invention is concerned with providing gas-produciug compositions which are plastic and so can be easily shaped during manufacture and can expand or contract due to temperature changes without an accompanying development of internal stress.

A particular object of the invention is to provide gasproducing charges which operate reliably and efiiciently under arduous physical conditions.

These and other objects are attained by providing in accordance with the invention, a plastic gas-producing composition which consists of an intimate mixture of constituents bound together by a polymeric organic viscoelastic liquid binder having a viscosity within the range 250,000 to 7,000,000 pulses at 25 C. The viscosity value is preferably Within the range 400,000 to 1,000,000 poises at 25 C.

A suitable binder is a linear polymeric hydrocarbon with side chains such as polyisobutene or polypropene. To the binder, a small proportion of an alkyl ester, preferably of lower freezing point, may be added as a plasticiser. Alternative binders which may be used include polyesters and polyglycols which have been chain extended by a di-isocyanate.

In accordance with a feature of the invention, in order to improve the action of the binder, there is incorporated in the charge composition a small quantity of a surface active or wetting agent which consists of a mixture of at least two compounds of which at least one is an organic sulphate, an inorganic salt of an esterified dibasic sulphonated acid, or an ester of a long chain fatty acid. Preferably each one of at least two of the compounds making up the surface active agent is either an alkali alkyl sulphate, an inorganic salt of a dialkyl sulphosuccinate, an alkyl oleate, or a polyoleate such as pentaerythritol dioleate. Examples of alkyl oleates which may be used are ethyl oleate and tetrahydrofurfuryl oleate.

A preferred surface active agent for incorporation in a charge composition comprises three compounds which are a polyoleate such as pentaerythritol dioleate, an alkyl oleate, and an inorganic salt of dialkyl sulphosuccinate.

An example of such a surface active agent is a particular surface active agent S which has a composition as follows:

Percent Peutaerythritol dioleate 30 Ethyl oleate 40 Sodium dioctyl sulphosuccinate 30 Either of the octyl groups in the last component may be in an isomeric form such as di-2-ethyl hexyl.

Preferred non-binder constituents are ammonium nitrate, ammonium perchlorate, ammonium picrate and oxamide. Ammonium nitrate and ammonium perchlorate are oxidants which aid the burning of the charge while ammonium picrate and oxamide are coolants which 3,073,730 ?atentod Jan. 15, 15963 Percent Ammonium perchlorate -12 Ammonium picrate 0-70 Surface active agent S s l Polyisobutenc 9-15 Catalyst 0-2 7 Part of the ammonium perchlorate portion may be replaced by ammonium nitrate as hereinafter described. A small part generally between about 10'l5%, of the polyisobutene may be replaced by ethyl oleate which aids the binding action of the polyisobutene and improves the physical properties of the composition at very low temperatures; the brittle point of the composition being lowered by about 10 '0 when about 2% of the ethyl oleate is added to the propellant.

The proportion of catalyst added is normally between 1 and 2%. The catalyst which may be a combination of two or more compounds may be added to increase the burning rate (when copper oxide, copper chromate and/ or chromium sequioxide), decrease the dependence of the rate of burning upon the temperature and pressure and improve the regularity of burning in general (when very finely divided titanium dioxide), or to suppress the flash produced by the propellant charge when burning (when a copper compound). I

Particulars of three specific compositions B, C and D which are within the formula of general composition A will not be given.

The first composition B is:

Percent Ammonium perchlorate 84.5 Polyisobutene v 12.5 Surface active agent S 1 Copper chromate 2 The ammonium perchlorate is milled to a specific surface of 8000 sq. cm./cu. cm. The burning rate at 1000 lb./sq. in. pressure is 1.53 in./sec.; and the pressure exponent of the rate of burning it (where the rate of burning is proportional to the nth power of the pressure) is 0. 5. The theoretical specific impulse is 232 lbs. sec./lb.

The polyisobutene and ethyl oleate are premixed and the ammonium perchlorate is milled to a specific surface of 2,000 sq. CI'fL/Cd. cm. prior to incorporation. The burning rate at 1000 lbs/sq. in. pressure is 0.53 in.'/sec.-: the pressure exponent of the rate of burning is 0.5; while the theoretical specific impulse is 223 lbs. sec/lb.

The third composition D is:

Percent Ammonium perchlorate 27 Ammonium picratc s 60 Surface active agent S 1 Polyisobutene 10.59 Ethyl oleate 1.41

The polyisobutene and ethyl oleate are premixed and the ammonium perchlorate is milled to a specific surface of 2000 sq. cm./cu. cm. prior to incorporation. The burning rate at 1000 lb./sq. in. pressure is 0.085 in./sec.; the pressure exponent of the rate of burning, 0.56; while the theoretical specific impulse is 184 lb. sec/lb.

On a weight basis, oxamide is three or four times as effective a coolant as ammonium picrate and when it is used in proportions up to about 15% in place ofpart of the ammonium picrate, low burning rate compositionsrare obtained.

A typical composition E containing oxamide is:

7 Percent Ammonium perchlorate -Q 39 Ammonium picrate 45 Oxamide Surface active agent 8 1 Polyisobutene 10 This composition has a burning rate of about 0.098 in./ sec. at a pressure of 1000 lb./sq. in. with a pressure exponent which is about 0.45 at 1000 lbs/sq. in. but which is very low or zero in the pressure range 1000 to 2000 lbs./ sq. in. The calculated specific impulse is 191 lb. sec./lb.

A continuous range of burning rates is obtainable between 1.53 and .07 in./sec. (at 1000 lb./sq. in. pressure) and any desired intermediate burning rate may be obtained by varying the oxidant particle size, by varying the ammonium picrate and/or oxamide content, or by altering the nature of the catalyst.

When part of the ammonium perchlorate is replaced by ammonium nitrate a certain minimum proportion of a material capable of vigorous self-sustained decomposition (for example, ammonium picrate or ammonium dichromate) is required. The quantity is depenent on the amount, and constitution, of the binder used. For example 8% ammonium picrate is required for 10% polyisobutene binder.

A plastic propellant composition F which employs a mixture of ammonium nitrate and perchlorate as oxidant is as follows:

Percent Ammonium nitrate 34 Ammonium perchlorate 40 Ammonium picrate 10 Surface active agent S 1 Polyisobutene 13 Ethyl 0163(6; 2

This composition has a burning rate of about 0.1 in./ sec. at a pressure of 1000 lb./sq. in. and with a pressure exponent of the rate of burning of 0.6. The calculated specific impulse is 205 lb. sec./ lb.

The plastic propellant material is made as an intimate uniform mixture of the various constituents, the solid constituents being in particle form. The burning rate of the compositions is very sensitive to the particle size and particle size distribution of the ammonium perchlorate and it is therefore important that these characteristics are maintained as constant as possible to a predetermined value.

A process for the manufacture of these plastic composit-ions will now be described.

As generally obtained, ammonium perchlorate must be initially broken down to a size, suitable for feeding to a mill. This breakdown is conveniently carried out on a machine consisting essentially of a large abrasive disc which rotates at about 30 rev/min. on a horizontal axis, inside a light metal casing. The pieces of ammonium perchlorate are fed into an inclined hopper and are grated against the abrasive disc. The resulting coarse powder is passed through an outletbetween the disc and the casing which is arranged to be about "A inch wide and restrict the maximum size of the product accordingly. All the machine bearings are sealed to prevent the ingress of amsize of the product from adry grinding mill varies with the moisture content of the feed. The drying may be conveniently carried out in any form of tray stove, preferably with hot air circulating at 140 to 160 F., the drying cycle being 12 hours.

The final reduction of the ammonium perchlorate to the desired particle size is carried out by a hammer mill such as a Harrison Carter Disintegrator or a Bramigk Mikro- Pulveriser. It is important that the operating conditions of the mill, such as the rate of feed and milling speed, are closely controlled and maintained constant in order that a final product having a particular particle size distribution as well as a particular average specific surface is produced.

The efiect of initial particle size of ammonium picrate or ammonium nitrate on the burning rate of the propellant composition is small compared with thatrof ammonium perchlorate and it is sufficient merely to prepare the ammonium picrate and ammonium nitrate if used by passing them through say a 60 B.S.S. mesh sieve.

Polyisobutene may be obtained in a number of grades varying from an easily pourable liquid to a'rubbery semisolid, depending on the molecular Weight. The viscosity of the polyisobutene used as a binder for the plastic propellant is preferably between 400,000 and 1,000,000 poises at 25 0., determined by falling sphere method in accordance with British Standard No. 188/1937. This viscosity has been chosen as it produces propellant material having the necessary rheological properties while the manufacturing temperatures are safely below temperatures at which decomposition or explosion occurs.

Polyisobutene which has a higher viscosity than that specified may be used by blending with other material having a viscosity value at the lower end of the range. Blending is carried out by mixing suitable proportions of the high and low viscosity material in a jacket'ted vessel for 12 hours, the temperature being kept at about C. by circulating low pressure steam in the jacket. The most suitable type of stirrer for this purpose has been found to be a spiral strip supported by horizontal struts from the central spindle, which runs just clear of the inner surface of the vessel.

In the manufacture of compositions containing ethyl oleate, thepolyisobutene is mixed with ethyl oleate at this stage.

The ammonium perchlorate, ammonium picrate, ammonium nitrate if used, and polyisobutene binder prepared as described are loaded into an ineorporator in weighed proportions in accordance with the particular propellant composition being made. The surface active agent S and any catalyst required are also added and the charge is mixed 'by the revolving blades of the incorporator for about two hours. The incorporator is preferably'maintained at about F. and it is advantageous for material adhering to the blades of the machine to be periodically scraped ofi and returned to the mix. A suitable machine is a Baker Perkins Imperial incorporator.

To improve the rheological properties of the propellant the material is then' passed between knurled differential rolls. In this process the binder is spread evenly over the surface of the crystals, forming a lubricating film which improves the flow properties of the propellant. The number of times the propellant is passed between the rolls affects the. particle size of the solid ingredients, which in turn determines the rate of burning of the propellant.

A machine on which this process may be performed is the Torrance Micro-Twin roll, both rolls being heated by hot water at 140 F. It consists essentially of two knurled bronze rolls which have a differential speed of 4 to 1, one of which is spring loaded. The gap between the rolls is adjustable and for the manufacture of plastic propellant charges is kept at 0.01 inch, while the load on the spring loaded roll is kept at the minimum. A drum conveyor rotates round the pair of ro1ls so that as material passes through the gap, it is picked up by the conveyor and refed to the top of the rolls.

A number of batches of composition delivered by the roll machine may be blended together if desired in the incorporator used for the initial mixing.

As an alternative to the incorporation and rolling process described, the constituents of the propellant charge may be blended in an incorporator with a high power input, for example rubber mixing types with high blade speeds, which are capable of producing acceptable propellants without the need for a rolling process. For a 250 lb. mix a power input of about 40 to 50 HP. is required.

To ensure satisfactory burning of the propellant charges produced by either of these methods as much air as possible is removed. This is done in a machine known as a deaerating pug mill as used extensively in the clay working industries; it consists of two jacketed screw extruders, separated by a vacuum chamber, the input extruder being at a higher level than the output extruder. The propellant charge is heated to 160 F. in a steam oven and is then fed to the machine which has been preheated by passing hot Water at160 F. through the jackets. The propellant is extruded in the form of cords through a die plate by the top screw into the vacuum chamber, which is maintained at a pressure of 1 to 5 mm. Hg by means of a vacuum pump. The air is removed from the propellant as it enters the vacuum chamber and the cords are cut off by a knife, carried by the top screw, which rotates just clear of the die plate. The de-aerated propellant charge falls into the lower screw and is then extruded through a die in consolidated form.

We claim:

1. A gas-producing composition consisting essentially of from 12% to 90% ammonium perchlorate, up to 70% ammonium picrate, 1% surface active agent consisting essentially of pentaerythritol dioleate, a compound from the group consisting of ethyl oleate and tetrahydrofurfuryl oleate, and an inorganic salt of dialkyl sulphosuccinate, in substantially equal amounts, 9 to 15% polyisobutene and up to 2% of a burning rate catalyst.

2. A gas-producing composition consisting essentially of 77% ammonium perchlorate, 7% ammonium picrate, 12.35% polyisobutene, 1.65% ethyl oleate, 0.5% copper oxide, 0.5% titanium dioxide and 1% surface active agent consisting essentially of pentaerythritol dioleate, ethyl oleate, and 30% sodium dioctyl sulphosuccinate; said percentage being by weight.

3. A gas-producing composition consisting essentially of 27% ammonium perchlorate, 60% ammonium picrate, 10.59% polyisobutene, 1.41% ethyl oleate and 1% surface active agent consisting essentially of 30% pentaerythritol dioleate, 40% ethyl oleate, and 30% sodium dioctyl sulphosuccinate; said percentages being by weight.

4. A gas-producing composition consisting essentially of 39% ammonium perchlorate, ammonium picrate, 5% oxamide, 10% polyisobutene and 1% surface active agent consisting essentially of 30% pentaerythritol dioleate, 40% ethyl oleate, and 30% sodium dioctyl sulphosuccinate', said percentage being by weight.

'5. A gas-producing composition consisting essentially of 34% ammonium nitrate, 40% ammonium perchlorate, 10% ammonium picrate, 13% polyisobutenes, 2% ethyl oleate and 1% surface active agent consisting essentially of 30% pentaerythritol dioleate, 40% ethyl oleate, and 30% sodium dioctyl sulphosuccinate; said percentages being by weight.

6. A gas-producing composition consisting essentially of 84.5% ammonium perchlorate, 12.5% polyisobutene, 2% copper chromate and 1% surface agent consisting essentially of 30% pentaerythritol dioleate, 40% ethyl oleate, and 30% sodium dioctyl sulphosuccinate; said percentages being by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,783,138 Parsons Feb. 26, 1957 2,791,883 Moore et al May 14, 1957 FOREIGN PATENTS 655,585 Great Britain July 25, 1951 742,283 Great Britain Dec. 21, "1955 OTHER REFERENCES Blatz: Industrial and Engineering Chemistry, vol. 8, No 4, April 1956, pp. 727-9.

Arendale: Industrial and Engineering Chemistry, vol. 8, No. 4, April 1956, pp. 725-6.

Mooret et 211.: Jet Propulsion, November 1956, pp. 965-968. 

1. A GAS-PRODUCING COMPOSITION CONSISTING ESSENTIALLY OF FROM 12% TO 90% AMMONIUM PERCHLORATE, UP TO 70% AMMONIUM PICRATE, 1% SURFACE ACTIVE AGENT CONSISTING ESSENTIALLY OF PENTAERYTHRITOL DIOLEATE, A COMPOUND FROM THE GROUP CONSISTING OF ETHYL OLEATE AND TETRAHYDROFURFURYL OLEATE, AND AN INORGANIC SALT OF DIALKYL SULPHOSUCCINATE, IN SUBSTANTIALLY EQUAL AMOUNTS, 9 TO 15% POLYISOBUTENE AND UP TO 2% OF A BURNING RATE CATALYST. 